Microporous filaments and fibers, and articles made therefrom

ABSTRACT

Fibers and filaments of an ultra-high-molecular-weight polyolefin that have a void volume greater than about 20% and are wettble. Wettability is imparted to the fibers and filaments by incorporating a filler into the composition that is hygroscopic, has a particle size less than about 10 microns in diameter and a surface area greater than about 30 square meters per gram. The preferred fillers are those that contain surface silanol groups. The fibers and filaments are formed by the process of preparing a mixture of an ultra-high-molecular-weight polyolefin, filler and plasticizer, metering the mixture to an extruder, heating and kneading the blend in the extruder, conveying the extrudate to a fiber or filament forming type die, expressing the extrudate through the die openings to form fibers or filaments, and extracting at least a portion of the extractable plasticizer to provide the desired porosity. The fibers and filaments may be formed into nonwoven webs directly by the melt blown or spun bonded process or by airlaying or wetlaying techniques. Such webs are particularly useful as a battery separator. The filaments may also be woven into fabric.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a division of Ser. No. 07/368,656, filed Jun. 20,1989, abandoned which, in turn, is a continuation-in-part of Ser. No.07/135,240, filed Dec. 21, 1987 abandoned.

BACKGROUND OF THE INVENTION

This invention relates to battery separators comprised of a web formedof a multiplicity of filaments and fibers formed of an ultrahighmolecular weight polyolefin that are microporous and wettable.

By "filament" it is intended to mean an essentially continuous strand ofmaterial; by "fiber" it is intended to mean a discontinuous strand ofmaterial.

By "strand", within the meaning of this invention, it is intended tomean the product resulting from extruding a molten blend of ultrahighmolecular weight polyolefin, filler, plasticizer and certain minoringredients through an orifice having a configuration capable of forminga filamentary extrudate, and subsequently extracting all or a part ofthe plasticizer.

By "microporous" it is intended to mean a filament or fiber that iscomprised of a plurality of interconnecting interstices that communicatewith the outside and inside of the filament or fiber body, theinterstices comprising a void volume of at least twenty percent of saidbody and preferably at least fifty percent of said body.

By "wettable" it is intended to mean the property of being able toabsorb water.

U.S. Pat. No. 4,422,993 discloses spinning solutions of ultrahighmolecular weight polyethylene to form filaments.

U.S. Pat. No. 4,545,950 discloses forming filaments, fibers, etc. byextruding a mixture of ultrahigh molecular weight polyethylene andparaffinic wax, cooling and stretching. The wax may be removed to form amicroporous article.

One of the features of this invention is to provide microporouspolyolefin filaments and fibers which exhibit good wettability.Polyolefin filaments and fibers are not wettable unless treated. Theprior art discloses using wetting agents to impart wettability topolyolefin fibers, such, for example, as disclosed in U.S. Pat. No.3,870,567; treating nonwoven webs with an aqueous bath comprising water,a surfactant, and colloidal silica such as is disclosed in U.S. Pat. No.3,985,580; coating nonwoven webs with a hydrophilic vinyl monomer andcatalyst such as is disclosed in U.S. Pat. No. 4,110,143; and treatmentwith corona discharge.

SUMMARY OF THE INVENTION

Filaments and fibers of ultrahigh molecular weight polyolefins areformed that have a void volume of at least twenty percent and arewettable. Wettability is imparted by incorporating a finely divided,hygroscopic filler material into the polyolefin extrusion mixture priorto extrusion.

The filaments and fibers are formed by extruding a mixture consistingessentially of the polyolefin, filler and an extractable plasticizerthrough an orifice capable of forming a filamentary extrudate, andsubsequently extracting at least part of the plasticizer by use of asolvent or nonsolvent for the plasticizer.

DESCRIPTION OF PREFERRED EMBODIMENTS

The polyolefin employed may be any crystalline homopolymer or copolymerof monoolefins having from two to four carbon atoms, i.e., ethylene,propylene and butylene. Preferred are homopolymers of ethylene andpropylene, and most preferred are high density polyethylene andsubstantially isotactic polypropylene homopolymers. The polyolefinshould have an ultra-high-molecular-weight ("UHMW"), i.e., a standardload melt index of less than about 0.04 per ten minutes, and preferably0, when measured in accordance with ASTM D 1238-70, an intrinsicviscosity greater than about 3.0 and, in the case of polyethylene, aninherent viscosity greater than about 5 dl/g (measured in decalin at135° C.). The preferred UHMW polyethylenes are those having a nominalweight average molecular weight between about 500,000 and about 5million when measured in accordance with ASTM D 4020-81 and a melt index(MFI) 190/5) less than about 0.01 g/10 min. Minor amounts of lowermolecular weight polyolefins may be blended therewith at lower fillerloadings. At higher filler loadings it is desirable to use UHMWpolyolefins in the higher molecular weight range of those disclosed asuseful herein.

It has been found that microporous polyolefin filaments and fibers canbe formed which exhibit good wettability by using as a filler admixedwith the polyolefin a material which exhibits good affinity for water,i.e., is hygroscopic in nature, and can withstand the temperature andpressure conditions existing in the extrusion process. However, thefiller should not be soluble in water to any substantial degree.

Although the term "filler" will be used herein to refer to the majornon-polyolefin component of the filaments and fibers of the presentinvention, it should be noted that there are several importantdistinctions between the term "filler" as used in the present inventionand the term "filler" as generally used in the polymeric fiber and filmart. The term "filler" as conventionally used means material added as areinforcement or as an extender. A reinforcing filler is used to improvemechanical or thermal properties of the polymer. Extenders are used toreduce cost or to improve processability of the polymer. The filler usedin the present invention is used primarily to provide enhancedmicroporosity and wettability to polyolefin filaments and fibers.

More importantly, however, the amount of filler relative to the amountof polymer used in the fibers of the present invention is far greaterthan the amount of filler material normally used in conventionalpolymeric filaments and fibers. Typically, fillers added to polymers informing conventional filaments and fibers are present in amounts, byweight, that are far less than the amount of polymer. In the filamentsand fibers of the present invention, the amount of filler is preferablygreater than the amount of polymer, i.e., a filler to polyolefin ratiogreater than about 1:1, and the amount of filler can be as great as afiller to polyolefin ration of about 20:1, i.e., the filler constitutesabout 90% by volume (about 95% by weight) of the filament or fiber. Forsome uses, the amount of filler may be as high as about 96% by volume ofthe filament or fiber. The reason why such high filler to polyolefinratio compositions are able to be formed into filaments and fibers isnot completely understood; however, it is known that such high fillerloadings cannot be formed into filaments and fibers having desirablestrength properties without using an ultra-high-molecular-weightpolyolefin as the polymeric component.

The following materials may be used as the filler material in thepresent invention: carbonaceous materials (e.g., carbon black andgraphite); metal oxides and hydroxides, such as those of silicon,aluminum, calcium, magnesium, barium, titanium, iron, zinc, and tin;metal carbonates, such as those of calcium and magnesium; minerals suchas mica, montmorillonite, kaolinite, attapulgite, asbestos, talc,diatomaceous earth and vermiculite; synthetic and natural zeolites;portland cement; precipitated metal silicates, such a calcium silicateand aluminum polysilicate; alumina silica gels; glass particles,including microbeads, microspheres, flakes and fibers; and salts, suchas molybdenum disulfide, zinc sulfide and barium sulfate.

The preferred filler materials are those that have surface silanolgroups, i.e., siliceous fillers, which can hydrogen bond to water, suchas silica, mica, montmorillonite, asbestos, talc, diatomaceous earth,vermiculite, synthetic and natural zeolites, portland cement, silicatesand polysilicates, alumina silica gels, and glass particles. Thepreferred siliceous filler is silica, and precipitated silica is thepreferred type of silica.

The filler should, desirably, have a high surface area, which means ithas either a small particle size or a high degree of porosity (i.e.,high surface area or pore volume), or both. The ultimate particle sizeof the filler can range from an average of about 0.01 micron to about 10microns in diameter; preferably, the average particle size is less thanabout 0.1 micron. The surface area of the filler can range from about 30to about 950 square meters per gram, and preferably is in the range offrom about 100 to about 500 square meters per gram. The pore volume isdesirably greater than about 0.075 cc per gram, and preferably fromabout 0.1 to about 0.4 cc per gram.

The surface area and pore volume of the filler can be measured using thenitrogen absorption method described by S. Brunauer, P. J. Emett, and E.Teller in the Journal of American Chemical Society, Vol. 6, page 308(1938), and commonly known as the BET method.

The preferred plasticizers used in the present invention may serve atleast three purposes: they enable ultra-high-molecular-weigthpolyolefins to be extruded with conventional extrusion equipment bylowering the melt viscosity; they are the component that is at leastpartially removed after formation of the filaments or fibers to impartporosity to the product; and, where all of the plasticizer is notremoved, the amount left in the final product may act in the classicalsense of a plasticizer to make the product less brittle. In addition,where the filaments or fibers are formed into a web for use as a batteryseparator, a small amount of plasticizer left in the filaments or fibersmay act to enhance oxidation resistance of the web to the electrolyte inthe battery.

Examples of suitable plasticizers for the present invention are organicesters, such as the sebacates, stearates, adipates, phthalates andcitrates; epoxy compounds, such as epoxidized vegetable oil; phosphateesters, such as tricresyl phosphate; hydrocarbon materials such aspetroleum oils; and natural oils such as tall oil and linseed oil. Thepreferred plasticizers are those extractable organic substances thathave a solubility parameter close to that of the polyolefin, preferablyin the range of from 7.3 to about 8.4. The most preferred plasticizersare petroleum hydrocarbon oils.

Although it is preferred to use a substantially water insolubleplasticizer, the present invention does not exclude the use of a mixtureof plasticizers, one of which is water soluble and is substantiallyremoved from the filaments or fibers after formation, so long as theplasticizer that is left in the final product to provide plasticizationis substantially water insoluble. Alternatively, a water solubleplasticizer could be used and the filaments or fibers treated with awater insoluble plasticizer after removal of the water solubleplasticizer by extraction where the presence of a plasticizer in thefinal product is desired. Suitable water soluble plasticizers includeethylene glycol, polyethylene glycol, polypropylene glycol, glycerol,and ethers and esters thereof; alkyl phosphates, such as triethylphosphate; polyvinyl alcohol; polyacrylic acid and polyvinylpyrrolidone.

In addition, although it is preferred to use as plasticizers extractableorganic substances having a solubility parameter in the range of about7.3 to about 8.4, extractable organic substances having a solubilityparameter greater than 8.4 may be used and extracted, followed byapplying a plasticizer having a solubility parameter between about 7.3to about 8.4 to the filaments or fibers where the presence of such aplasticizer in the final product is desired to perform one of thefunctions described above.

Other, conventional, additives may be added to the extrusion mixture,such as antioxidants, colorants and lubricants.

The ultra-high-molecular-weight polyolefin component should comprisebetween about 10 and about 90 percent by volume (between about 5 and 80percent by weight) of the filaments or fibers, preferably between about40 and about 60 percent by volume for uses where resistance to rapidwettability of the web formed from the fibers and filaments is desiredand preferably between about 10 and about 40 percent by volume wheresuch rapid wettability is desired. For some uses, the amount of UHMWpolyolefin may be as low as about 4% by volume.

The filler should comprise between about 10 and about 90 percent byvolume of the filaments or fibers, preferably between about 40 and about60 percent by volume for uses where resistance to rapid wettability ofthe web formed from the fibers and filaments is desired and preferablybetween about 60 and about 90 percent by volume where such rapidwettability is desired. For some uses, the filler may be as high asabout 96% by volume of the filaments or fibers.

Where one of the product enhancing characteristics imparted to thefilaments or fibers by the plasticizer described above is desired, theplasticizer component should comprise between about 1 and about 15percent by volume of the filaments or fibers, preferably between about 1and about 10 percent by volume. For many uses however, it may bedesirable to remove substantially all of the plasticizer from thefilaments or fibers so that there is substantially no plasticizerremaining.

In the process of the present invention, a blend is introduced into theextruder which comprises from about 5 to about 65 percent by volume ofthe ultrahigh molecular weight polyolefin component, about 5 to about 60volume percent of the filler component and about 20 to about 80 percent(preferably about 50 to about 80 percent) by volume of the plasticizercomponent. Other minor conventional additives may be present in theextrusion mixture in amounts recommended by their manufacturers.

Where the plasticizer is a liquid, as, for example, a petroleumhydrocarbon oil, it is preferred to add a portion of the plasticizer tothe dry ingredients during mixing, and to add the remainder of theplasticizer directly to a feed port of the extruder together with the"dry" ingredients. The extrusion mixture "dry" components may be mixedprior to introduction to the extrusion operation by any conventionalmixing means although it is important that a substantially uniformmixture be obtained. The amount of plasticizer added at the mixing stageshould preferably be as much as can be added without losing a somewhatpowdery consistency.

It should be noted, however, that it is sometimes convenient to"compound" the extrusion mixture prior to extruding the mixture intofibers or filaments, i.e., to follow the above procedure but extrude themixture through a compounding die and chopping the resulting strandsinto pellets. Where the mixture is compounded before fiberizing, all ofthe plasticizer may be added to the compounded pellets, and when thepellets are metered to a fiberizing extruder/die, no additionalplasticizer need be added.

The extrusion mixture is metered to any conventional extrusion deviceknown to be suitable in the art of filament and fiber formation. Screwextruders having two, counter-rotating or co-rotating, screws arepreferred. There are a number of manufacturers supplying such deviceswhich come in various sizes dependent upon the throughput of materialdesired. The heating zones in the extruder barrel are heated totemperatures to obtain the desired degree of plasticization, whichdepends upon the particular polyolefin selected and the formulation fedto the extruder. For ultrahigh molecular weight polyethylene, it isgenerally desired to maintain the barrel at temperatures between about200° and about 250° C.

The output of the extrusion device is fed to a die suitable for formingfilaments or fibers, such as a spinneret type die. The optimumtemperature of the die depends upon the particular extrusion mixtureemployed but it is generally preferable to maintain the die at about thesame temperature as that of the extruder barrel. Such dies are wellknown in the art, and may include manifolds on one or both sides of thedie orifices for directing a hot gas stream against the extrudate at anangle designed to attenuate the filaments or fibers being extruded. Thetemperature of such hot gas stream depends upon the polyolefincomposition being extruded, and will generally be in the range normallyused for conventional polyolefins in the particular filament formingprocess chosen. In the melt blowing process, for example, airtemperatures will generally be about the same as the die temperature.

Alternatively, and especially where continuous filaments are desired,individual filaments or bundles of filaments extruded from the die maybe fed into a separate attenuating device employing a heated gas streamto attenuate the filaments.

Attenuation may be accomplished by mechanical tension (e.g., godetrolls) applied to the filaments extruded from the die while thefilaments are still at an elevated temperature, or subsequently afterthe filaments have cooled. It is preferred to perform attenuation offilaments after the filaments have been subjected to the extractionstep. Attenuation of filaments tends to make them more supple andimproves tensile strength and modulus of elasticity. Attenuation mayalso increase void volume by introducing stretch induced micropores ofthe type formed in stretched polyolefin films, such as is described inU.S. Pat. No. 4,359,510. However, for certain uses it may be desirableto omit attenuation.

After filament or fiber formation, the plasticizer is extractedtherefrom. The plasticizer may be extracted from the filaments or fibersbefore or after they are formed into a web. In the melt blown or spunbonded processes where a web is formed directly from the extruded fibersor filaments, the plasticizer is extracted after formation of the web.If two or more plasticizers are employed, multistage extraction may berequired, particularly if one plasticizer is water insoluble and one iswater soluble.

The solvent chosen to extract the plasticizer depends upon the nature ofthe plasticizer. Where a petroleum hydrocarbon oil is to be extracted,the following solvents are suitable: chlorinated hydrocarbons, such astrichloroethylene, 1,1,1-trichloroethane, methylene chloride,perchloroethylene, tetrachloroethylene, carbon tetrachloride, etc.;hydrocarbon solvents, such as hexane, benzene, petroleum ether, toluene,cyclohexane, etc.; and chlorofluorocarbons, such astrichlorotrifluoroethane. If a water soluble plasticizer such aspolyethylene glycol is to be extracted, suitable solvents include:water; alcohols, such as methanol and ethanol; acetone; etc.

The temperature at which the extraction is carried out can vary fromambient up to the melting point of the polyolefin. The extraction timedepends on the extraction temperature, and the time and temperature arechosen so that the desired amount of plasticizer is removed from thefilaments or fibers. As mentioned previously, it has been found that forcertain uses it is desirable to leave a plasticizing amount of theplasticizer in the final product. Therefore, extraction would be carriedout under temperature and time conditions such that the desired amountof the plasticizer is left in the product. Alternatively, although lessdesirable, all of the plasticizer may be removed and the desired amountadded to the filaments or fibers in a subsequent treatment step. Theparticular means employed to carry out the extraction is not part of thepresent invention. The extraction may be carried out on a batch basis oron a continuous basis by passing the filaments or fibers, or a webformed therefrom, through a liquid and/or vapor bath of the extractionmedia, generally in a countercurrent extraction manner. The extractionmedia and the plasticizer may then be separated and recovered bydistillation or other suitable separation means.

It has been found desirable to dry the extracted fibers or filaments, orwebs formed therefrom, at an elevated temperature after plasticizerextraction. Subjecting the extracted fibers, filaments or webs to anelevated temperature appears to aid in developing enhanced wettability.Temperatures between about 212° F. to about 280° F. are useful in thisregard.

The fibers of the present invention may be formed by conventional fiberforming techniques known in the art. For example, fibers may be formedby melt blowing in which the compositions disclosed herein are fed to anextruder for plasticization, the extrudate from the extruder fed to aspinneret type die head containing a plurality of small die openings,and feeding the material from the die openings into a gas stream toattenuate the extrudate and form the fibers. The fibers may be collectedupon a moving foraminous collection device as a mat. Suitable devicesare described in U.S. Pat. No. 3,650,866 and U.S. Pat. No. 3,947,537.

Filaments may be formed by extruding the extrudate through a die orificeof a suitable type, such as a spinneret type die or, in the case ofmonofilaments, through a plate containing many small holes, andattenuating the filaments by conventional mechanical or pneumatic meanseither while the filaments are in a heated state or after cooling, andpreferably after extraction. One such process of forming continuousfilaments is described in U.S. Pat. No. 3,692,618. The filaments may, ifdesired, be chopped into staple fibers to provide fibers having a largerdiameter than is usually formed by melt blowing.

The filaments and fibers of the present invention have a void volumegreater than about 20%, preferably greater than about 50%, and as highas about 80% to about 95%. Void volume is determined by removing theplasticizer by solvent extraction and determining the amount ofextracted oil that was present in the filaments or fibers.

The average pore diameter of the pores in the fibers and filaments ofthe present invention is less than about 1.0 micron, and at least about90% of the pores have a pore diameter of less than about 0.5 micron. Theaverage pore diameter can range down to about 0.1 micron or less. Porediameter is measured by conventional mercury intrusion techniques. Thepores or porosity of the fibers and filaments of this invention aresometimes referred to herein as "micropores" or "microporosity" and thepores or porosity created by the interfiber or interfilament intersticesof woven or nonwoven sheets made from them referred to as "macropores"or "macroporosity".

For fibers formed by the melt blowing process, the average diameter isless than 10 microns, and generally ranges between about 1 and about 10microns.

The "coarseness" of the fibers of the present invention is less than 1decigrex, and generally less than about 0.5 decigrex. By comparison,ordinary polyolefin melt blown fibers have a coarseness greater than 1decigrex. Generally speaking, the coarseness of the fibers of thepresent invention will be approximately 20% to about 50% that of anonporous fibers of corresponding dimensions, depending upon the voidvolume. "Coarseness" is measured by TAPPI Method No. T234 SU-67; thedecigrex unit is measured as weight of fibers in milligrams per 100meters of the fibers.

The fibers of the present invention may be used to form nonwoven websdirectly as part of the melt blown process as, for example, described inU.S. Pat. No. 3,947,537. The nonwoven webs thus formed may becalendered, either before and/or after plasticizer extraction, tocontrol the thickness and porosity of the web to desired levels.Alternatively, nonwoven webs can be formed in a seperate operation byconventional airlaying techniques either with adhesives to enhance fiberto fiber bonding or by heating and/or pressing such webs to cause fiberto fiber bonding.

The fibers may also be formed into webs by the wetlaid processes used incellulosic paper manufacture. The fibers, being wettable, can be formedinto a web eithr alone or in admixture with cellulosic fibers, or otherfibers conventionally formed into webs by the wetlaid process, such asglass fibers, synthetic pulp, synthetic polymeric fibers, etc. Althoughthe fibers of the present invention are wettable, they are hydrophobicat lower filler loadings and it is, therefore, desirable to employ adispersing agent to aid in the uniform dispersion of such fibers inaqueous slurries. Suitable dispersing agents include anionicsurfactants, such as alkali salts of higher fatty acids, alkylsulfonicacid salts, alkylaryl sulfonate salts and sulfosuccinate ester salts. Asan additive to cellulosic fibers, the fibers of the present inventionenhance optical properties, such as brightness and opacity, of paperformed from such a blend. Such paper has enhanced printability with inksof all types (including water-based, oil-based and organicsolvent-based), particularly where the fibers of the present inventionconstitute all or a substantial portion of the fibers in the paper web.The present fibers also impart water resistance to such paper webs atlower filler loadings, as discussed above.

Webs formed from fibers of the present invention, whether formeddirectly (such as by the melt blown or spun bonded processes), or by anairlaid or wetlaid technique, and whether or not they are mixed withcellulosic or other types of fibers, exhibit enhanced printability withinks of all types. Such webs are either water resistant (where thefibers of the present invention are present as the sole or major portionof the total fiber content and have lower filler loadings) or exhibitenhanced water resistance (where the fibers are present to a lesserextent and have lower filler loadings). Such webs are rapidly wettablewhere the fibers have a higher filler loading, as discussed above.

The continuous filaments of the present invention may be formed intospunbonded webs by forming a web of the filaments ranging from less thanabout 0.5 to about 10 microns in thickness and bonding under heat andpressure by conventional techniques, such as those used to form Tyvek(trademark of E. I.Dupont) or used in the Docan process (LurgiMineroltechnik GmbH).

The continuous filaments of the present invention may also be woven intofabric webs by conventional weaving techniques.

Whether the continuous filaments are formed into webs by the spunbondedor similar techniques, or woven into fabrics, they offer the sameadvantage relative to dyeability discussed above relative to nonwovenwebs. Prior art polyolefin filaments are not easily dyeable, andgenerally colorants are added to the extrusion mixture prior toformation of filaments. The filaments of the present invention aredyeable with a wide variety of conventional dyes normally used withnatural or other dyeable synthetic filaments. The dyeability of thefilaments of the present invention can be tailored to individual dyesystems by adjusting the amount of filler loading to provide optimumsurface tension of the web relative to the dye selected.

Although generally formed in dies having round or oval orifices incross-section, the fibers and filaments of the present invention may beformed in dies whose orifices have almost any cross-sectional shape,including those designed to form hollow filaments or fibers. Althoughthe morphology of the filaments or fibers of the present invention hasnot been studied in great detail, electron microphotographs atmagnifications up to 8,000X show that the fibers have an extremelyrough, porous surface.

The fibers and filaments of the present invention may be used for anyuse to which fibers and filaments of other materials are currently used.Because of their porosity, they form articles of lighter weight thansimilar, solid fibers or filaments. This property, together with theproperties of water resistance at lower filler loadings and excellentdyeability, makes woven and nonwoven fabrics and webs made therefromparticularly useful for tenting and clothing. The fibers and filamentsof the present invention, and fabrics and webs made therefrom, also makeexcellent filter media, and can also be used as the substrate for timedrelease of pharmaceuticals, agricultural and other chemicals.

A particularly good use for the woven and nonwoven webs made from thefilaments and fibers of the present invention are as battery separators.It is known to make battery separators from nonwoven webs ofpolyolefins, such as is disclosed in U.S. Pat. No. 3,870,567, and it isknown to make battery separators from microporous sheets, such as isdisclosed in U.S. Pat. No. 3,351,495. However, it was not known beforethe present invention to form battery separators of fibers or filamentsthat were microporous and wettable without the aid of surfactants.

The use of microporous filaments or fibers that are wettable to form thebattery separator provides greatly enhanced ability to absorbelectrolyte over prior nonwoven web type separators or over priormicroporous sheet separators. This is due to the fact that separatorsformed of the filaments or fibers of this invention exhibit bothmicroporosity and macroporosity. This property is particularly useful informing "starved electrolyte" (recombinant) type batteries where theability of the separator to absorb and retain electolyte is criticalsince the only electrolyte present is that absorbed by the separator andplate.

The woven or nonwoven webs may be used as separators by themselves orattached to another separator material, such as the sheet type separatordisclosed in U.S. Pat. No. 3,351,495. Such nonwoven web laminates permitthe use of much thinner sheet separators to which the nonwoven web isattached (such as the sheet separator of the aforementioned '495 patent)since the nonwoven web acts as a reinforcing layer. Alternatively, oneor both surfaces of a web formed from the filaments or fibers of thepresent invention may be wholly or partially fused by heat and/orpressure to form a film-like surface.

The separators of the present invention may be substituted for glassfiber type separators currently used in both flooded cell and starvedelectrolyte load acid batteries, such as those disclosed in numerousissued patents as, for example, U.S. Pat. Nos. 4,072,802, 4,153,759,4,414,295 etc., and are superior to such glass fiber separators due tothe microporosity of the fibers and filaments of the present inventionand the more favorable economics of being able to form the separator webat less energy cost and a web that weighs less than an identically sizedglass fiber web.

It is preferred to make the separators from the fibers and filaments ofthis invention at thicknesses of less than about 15 mils, and preferablyfrom about 1 to about 10 mils for use in flooded cell type batteries.For starved electrolyte type batteries, and certain flooded cell typebatteries, the thickness of the separator is usually greater, preferablybetween about 15 and about 120 mils. Such separators exhibit anelectrical resistance of less than about 1.5 milliohms per square inchper mil thickness.

The following examples illustrate specific ways of practicing thepresent invention, but are not to be construed as limiting the scope ofthe invention:

EXAMPLE 1

A mixture containing 37 pounds of ultrahigh molecular weightpolyethylene (Himont 1900 UHMW Polymer), 137 pounds of hydrated,amorphous silica (Hi-Sil 233 manufactured by PPG Industries), 0.6 poundsof Irganox B 215 (an antioxidant/stabilizer manufactured by Ciba-Geigy),and 0.6 pounds of Petrac CZ-81 (a lubricant manufactured by Desoto,Inc.) is thoroughly blended in a Littleford high intensity mixer for 2minutes. To the mixer was added 27 gallons of Shellflex 412 (a petroleumhydrocarbon oil manufactured by Shell Oil Company) at a temperature ofabout 125° F., and the resulting mixture blended for an additional fiveminutes. The resulting mixed blend is placed into a Marion continuousblender and blended until fed to the extruder. The blend is metered tothe feed port of a Leistritz Model ZSE 96 extruder at the rate of 450pounds per hour. At the same time, Shellflex 412 is continuously meteredinto the feed port of the extruder at the rate of 50 pounds per hour.The barrel is heated to a temperature of 220° C. The extrudate is fed toa die having the design disclosed in U.S. Pat. No. 3,947,537 maintainedat a temperature of 220° C. Hot air (600° F.) is fed into the manifoldsurrounding the die holes at a rate that provides good fiber formation.The fibers thus formed are collected on a rotary screen located 24inches from the die. A representative sample weighing 100 grams of thecollected fibers is placed into the 12 liter flask of a Soxhletextractor containing 6 liters of 1,1,1-trichloroethane and extracted for15 minutes at 72° C. The extracted fibers have a void volume ofapproximately 60%.

EXAMPLE 2

A hand sheet is prepared from the fibers of Example 1 in accordance withTAPPI T205 m-58. The resulting handsheet exhibits good formation, isvery white in appearance and holds together nicely.

EXAMPLE 3

The procedure of Example 1 using the same composition is repeated exceptthat the heated air to the manifolds is turned off. The continuousfilaments of extrudate thus formed are allowed to cool. Several lengthsof filaments approximately one meter in length are cut and extracted ina Soxhlet extractor as in Example 1. After extraction, the filaments arestretched (attenuated) to a length of approximately two meters. Thediameter is reduced as a result of this stretching and very whitecontinuous filaments are obtained which are wettable.

EXAMPLE 4

Example 1 is repeated except that the collector is located approximately10 inches from the die. The resulting mat is cut into a sheetapproximately 6×6 inches in size. The sheet is extracted in a Soxhletextractor as in Example 1. The extracted sheet is placed between twoplates of a press approximately 10×10 inches in size at a platetemperature of about 280° F. The plates are compressed to a gap of about10 mils and kept closed for about 20 seconds. The resulting nonwoven webis white in appearance and is wettable.

EXAMPLE 5

The nonwoven web of Example 4 is measured for electrical resistance in aPalico tester involving a ten minute soak in boiling (100° C.) water anda twenty minute soak in 1.280 specific gravity sulfuric acid at 26° C.The electrical resistance is 10 milliohms per square inch.

EXAMPLE 6

A mixture containing 3.3 pounds of ultrahigh molecular weightpolyethylene (Himont 1900 UHMW Polymer), 11.5 pounds of hydrated,amorphous silica (Hi-Sil 233 manufactured by PPG Industries), 0.06pounds of Irganox B 215 (an antioxidant/stabilizer manufactured byCiba-Geigy), and 0.06 pounds of Petrac CZ-81 (a lubricant manufacturedby Desoto, Inc.) was thoroughly blended in a Littleford high intensitymixer for 2 minutes. To the mixer was added 3.8 gallons of Shellflex 412(a petroleum hydrocarbon oil manufactured by Shell Oil Company) at atemperature of about 70° F., and the resulting mixture blended for anadditional five minutes. The blend was metered to the first feed port ofa Betol Model BTS 40 extruder at the rate of 31 grams per minute. At thesame time, Shellflex 412 was continuously metered into the second feedport of the extruder at the rate of 11 grams per minute. The barrel washeated to temperatures of 195° C. to 240° C. The extrudate is fed to adie having the design disclosed in U.S. Pat. No. 3,947,537. Thenosepiece of the die was ten inches in length and had 100 holes each ofwhich had a diameter of 0.025 inch. The die was maintained at atemperature of 236° C. to 315° C. Hot air at 315° C. is fed into themanifold surrounding the die holes at a rate that provides good fiberformation. The fibers thus formed are collected on a rotary screenlocated approximately 36 inches from the die. A representative sampleweighing 100 grams of the collected fibers is placed into a 12 literflask of a Soxhlet extractor containing 6 liters of1,1,1-trichloroethane and extracted for 15 minutes at 72° C. Theextracted fibers have a void volume of approximately 60%.

EXAMPLE 7

A number of blends having the percent by weight compositions set forthbelow were prepared:

    ______________________________________                                        Example   Polyethylene.sup.1                                                                       Silica.sup.2                                                                            Oil.sup.3                                                                           Other.sup.4                              ______________________________________                                        7A        7.6%       30.4%     61.9% 0.3%                                     7B        4.2%       33.7%     61.9% 0.3%                                     7C        3.4%       34.4%     61.9% 0.3%                                     7D        2.9%       34.9%     61.9% 0.3%                                     7E        2.5%       35.3%     61.9% 0.3%                                     ______________________________________                                         Footnotes:                                                                    .sup.1 7A: Hoechst 7255; 7B-7D: Hoechst 412; 7E: Hoechst 413                  .sup.2 PPG 86C (Note: Percent by weight based upon ambient, not bone dry      weight.)                                                                      .sup.3 Shellflex 412                                                          .sup.4 Irganox B 215 and Petrac CZ81 (0.15% each)                        

Each of these blends was metered into the first feed port of a BetolModel BTS 40 twin screw (co-rotating) extruder at the rate ofapproximately 38 grams per minute. At the same time, Shellflex 412 atroom temperature was continuously metered into the second feed port ofthe extruder at the rate of 13 grams per minute. The extruder barrel washeated to temperatures of 160° C. to 270° C. The extrudate from theextruder was fed to a die having the design disclosed in U.S. Pat. No.3,947,537. The nosepiece of the die was ten inches in length and had 200holes each having a diameter of 0.015 inch. The die was maintained at atemperature of 260° C. Hot air at a temperature of 260° C. was fed intothe manifold surrounding the die holes at a pressure in the manifold of10 psi. The fibers were blown onto a rotary collector and formed intononwoven webs. Samples of the webs were extracted with1,1,1-trichloroethane to remove substantially all of the oil and driedin an oven at 270° F. for one hour. The webs all had good strength andwere white in appearance.

EXAMPLE 8

The webs prepared in example 7 were tested for wettability, as follows:Aqueous solutions of methanol were prepared at concentrations between 5%and 45% by weight. One hypodermic syringe was filled with eachconcentration. Starting with the webs formed from fibers with the lowestfiller loading, a drop of water and a drops of the 45%, 40% and 35%solutions (at room temperature, 68° F.) were placed on each of the websto determine if it immediately soaked in or formed a ball on the websurface. (It was found that the 40% methanol solution wet all webs,regardless of filler loading.) The results were as follows:

    ______________________________________                                        Example No.                                                                            Water Wets?  Lowest Methanol Wetting (%)                             ______________________________________                                        7A       No           40%                                                     7B       No           30%                                                     7C       No           20%                                                     7D       No           10%                                                     7E       Yes           0%                                                     ______________________________________                                    

It can be seen from the foregoing that as the filler loading wasincreased the webs became more wettable until, at a filler loading of35.3% by weight (20% by volume) (silica to polymer weight ratio of 14:1)the webs readily wet with water. The methanol concentration at which thewebs wet may be correlated to the surface tension of the web, and, thus,webs may be designed to have a surface tension (degree of wettability)to whatever extent it is desired.

EXAMPLE 9

Example 7A was repeated using a number of different silicas substitutedfor that used in the example. The silicas used were:

    ______________________________________                                        Example     Silica         Manufacturer                                       ______________________________________                                        9A          VN3 SP         Degussa                                            9B          Cab-O-Sil S-17 Cabot                                              9C          LO-VE2         PPG                                                9D          Aerosil OX 50  Degussa                                            9E          Aerosil 380    Degussa                                            ______________________________________                                    

The extracted webs were strong and white in appearance.

We claim:
 1. A battery separator comprised of a web formed of amultiplicity of microporous fibers or filaments,the composition of saidmicroporous fibers or filaments being comprised of between about 4% toabout 90% by volume of an ultrahigh-molecular weight polyolefin selectedfrom the group consisting of homopolymers and copolymers of ethylene,propylene and butylene; between about 10% to about 96% by volume of ahygroscopic filler having a particle size less than about 10 microns;said microporous fibers or filaments having a void volume greater thanabout 20%; said web having: a thickness of between about 1 and about 120mils; a macroporosity formed by the interstices between said microporousfibers or filaments; and an electrical resistance of less than about 1.5milliohms per square inch per mil thickness.
 2. The battery separator ofclaim 1 having a thickness between about 1 and about 15 mils.
 3. Thebattery separator of claim 1 having a thickness between about 15 andabout 120 mils.
 4. The battery separator of any of claims 1, 2 or 3wherein the ratio of filler to polyolefin is between about 1:1 and about20:1.
 5. The battery separator of claim 1 wherein the filler comprisesbetween about 60% and about 96% by volume.
 6. The battery separator ofany of claims 1, 4 or 5 wherein the filler is silica.
 7. The batteryseparator of any of claim 1, 4, 5 or 6 wherein the polyolefin isultra-high-molecular-weight polyethylene.
 8. A battery separator for usein a recombinant type lead acid battery comprising a web formed of amultiplicity of microporous fibers or filaments,the composition of saidmicroporous fibers or filaments being between about 4% to about 90% byvolume of an ultra-high-molecular weight polyethylene; between about 10%to about 96% by volume of silica having a particle size less than about10 microns; said web having: a thickness of between about 15 and about120 mils; a macroporosity formed by the interstices between saidmicroporous fibers or filaments; and an electrical resistance of lessthan about 1.5 milliohms per square inch per mil thickness.
 9. Thebattery separator of claim 8 wherein the silica content of themicroporous fiber or filament is between about 60% and about 96% byvolume.
 10. The battery separator of claim 8 or claim 9 wherein theaverage diameter of the microporous fibers or filaments is less thanabout 10 microns.